The present invention relates to chip testers. More particularly, the present invention relates to methods of extending the operational period of a heat-exchanger in a chip tester, where the heat-exchanger is of the type that includes an electric heater and a heatsink that are joined together with a layer of attach material.
Within the chip tester, the heat-exchanger contacts an integrated circuit chip and maintains the chip's temperature near a set point while the chip is tested. A prior art chip tester which has a heat-exchanger with an electric heater and a heatsink is disclosed in U.S. Pat. No. 5,821,505.
For ease of reference, FIG. 1 of the 505 patent is reproduced herein as FIG. 1 and labeled prior art. In that figure, reference numeral 11 identifies the integrated circuit chip which is to be tested while its temperature is maintained near the set point. The chip 11 may be packaged in ceramic or plastic, or it may be unpackaged.
The chip 11 has dozens of input/output terminals, and some of those terminals are shown in FIG. 1 as being coupled to the signal lines 12a, 12b, and 12c. “TEST-IN” signals are received by the chip 11 on the signal lines 12a; “TEST-OUT” signals are sent by the chip 11 to the signal lines 12b; and “TEMP” signals are sent by the chip 11 to the signal lines 12c. The TEMP signals indicate the temperature of the chip 11, and they originate from a temperature sensor which is integrated into the chip 11.
All of the remaining components 12–17 in FIG. 1 comprise a portion of the chip tester which keeps the temperature of the chip 11 near the set point while that chip is being tested. Each component 12–17 is described in TABLE 1 of patent '505, which is reproduced below.
TABLE 1COMPONENTDESCRIPTION12Component 12 is a printed circuitboard which physically holds thecomponents 11, 16, 17 and 18.Also, the printed circuit board 12contains several sets of signaland power lines 12a–12g.13Component 13 is a thin, flatelectric heater which has two majorfaces 13a and 13b that lie againstcomponents 11 and 14 respectively.A variable amount of electricalpower Ph is supplied to the heater13 via two wires 13c, and thatpower is dissipated as heat withinthe heater. One embodiment of theheater 13 is comprised of aluminumnitride ceramic in which electricalresistors (not shown) are uniformlyintegrated for converting the powerfrom the conductors 13c to heat.14Component 14 is a liquid cooledheatsink that has a hollow base 14ain which cooling fins (not shown)are disposed. A liquid coolant 14benters the base 14a from a tube14c, and that liquid coolant exitsthe base via another tube 14d.This coolant 14b is circulatedthrough the base 14a at a constantflow rate by a pump (not shown) andheld at a constant temperature TL.15Component 15 is an electroniccontrol circuit which sends thevariable amount of electrical powerPh to the electric heater 13. Thiscontrol circuit 15 consists of apower regulator 16 and a variablepower supply 17.16Component 16 is a power regulatorwhich is coupled to three sets ofsignal lines 12c, 12d, and 12e.The TEMP signals which indicate thepresent temperature Td of the chip11 are received on the signal lines12c, and SET-POINT signals whichindicate the set point temperaturefor the chip 11 are received on thesignal lines 12d. Based on thosetwo temperatures and their rate ofchange, power regulator 16generates control signals CTL onthe signal lines 12e which indicatethe amount of power that should besent to the heater 13 such that thetemperature of the chip 11 staysnear the set point.17Component 17 is a variable powersupply which is coupled to thesignal lines 12e and two sets ofpower lines 12f and 12g. On thesignal lines 12e, the controlsignals CTL from the powerregulator 16 are received, and onthe power lines 12f, a supplyvoltage +V and ground are received.In response to the CTL signals, thepower supply 17 sends the variableamount of power Ph on the powerlines 12g as a portion of the powerwhich is available from the supplyvoltage +V.18Component 18 is a connector whichintercouples the heater wires 13cto the variable power supply.
In operation, the chip 11 is tested by sending it the TEST-IN signals on the signal lines 12a, and by examining the chip's response via the TEST-OUT signals on the signal lines 12c. While that occurs, the power dissipation in the chip 11 varies because many transistors within the chip 11 turn-on and turn-off whenever the TEST-IN and TEST-OUT signals change. As the power dissipation of the chip 11 increases the chip temperature tends to increase; and vise-versa.
The TEMP signals from the chip 11 are sent on the signal lines 12c to the power regulator 16 where they are compared to the SET-POINT signals on the signal lines 12d. When the temperature of the chip 11 is too cold relative to the set point temperature, then the regulator 16 increases the heater power Ph via the control signals CTL. Conversely, when the temperature of the chip 11 is too hot relative to the set point temperature, then the regulator 16 decreases the heater power Ph.
Due to the above described operation of the power regulator 16, the testing of just a single chip typically subjects the heater 13 to thousands of different changes in temperature. Thus, as multiple chips are tested in the FIG. 1 chip tester over time, the total number of temperature changes to which the heater 13 is subjected can easily exceed one million.
The present inventors have closely analyzed the above temperature changes in the FIG. 1 tester to see if they have any long term adverse effect on the tester. What the present inventors found is that as the number of chips which have been tested increases, the thermal resistance increases through an epoxy layer which joins the heater 13 to the heatsink 14. This epoxy layer is shown and described as item 102 in FIG. 18 of the '505 patent, and that figure is reproduced herein as FIG. 2.
Further the present inventors have determined from their analysis that the above increase in thermal resistance is caused by microscopic stress cracks that are induced in the epoxy layer 102 when the heater 13 is subjected to a large number of temperature changes while multiple chips 11 are tested. These stress cracks are schematically shown in FIG. 3 where they are indicated by reference numeral 200. The present inventors have determined that the stress cracks 200 occur because the heater 13 and the heatsink 14 expand at different rates when the above temperature changes occur.
The presence of the stress cracks 200 is a serious problem because they eventually cause the thermal resistance through the epoxy layer 102 to become so large that the temperature of the chip 11 cannot be kept at the set point. When that occurs, the entire heat-exchanger (i.e.—the heater 13 and the attached heatsink 14) needs to be removed from the tester and replaced. But replacing the entire heat-exchanger is expensive, and it also causes downtime on the chip tester.
Accordingly, a primary object of the present invention is to provide a method of extending the operational period of the heat-exchanger in a chip tester, which completely avoids the above need to replace the heat-exchanger.